We have determined hydration thermodynamics, deltaH(std,298), deltaS(std,298), and deltaG(std,298), for a family of alkyl ammonium ions, CnH2n+1NH3+ for n between 1 and 8 carbons, using electrospray ionization-equilibrium gas phase ion molecule reactions at equilibrium. The hydration thermodynamic values were calculated from equilibrium constants measured under conditions of ion source water partial pressures ranging between 0 and 100 mtorr, equivalent to 0-1% of total source pressure, and ion source temperatures ranging between 5 and 65C. Equilibrium ion intensity measurements were made for at least 4 hydration states, i.e., zero through 3 water molecules associated with the core ion, at each of the 60 combinations of water partial pressures and temperatures covering the ranges of experimental variables. Results of our measurements are in close agreement with published data for some of the alkyl ammonium species for the hydration states where common data are present. Of much greater importance is that we have been able to establish for the first time a measurement of the entropic strength of a single intermolecular hydrogen bond. The values for deltaS(std) are sufficiently precise that we are able to conclude that there is an average net loss in entropy of 3 cal/deg.mole in adding the second water to an ion already having one water present, i.e., delta-deltaS(std) = 3 cal/deg.mole significant at p < 0.04. The net decrease in entropy is consistent with a gain in intermolecular ordering, that is the formation of a water - water - N+ hydrogen bond. Furthermore, our studies clearly show that 1) addition of the first water to a bare ion is apprecibly more exothermic than addition of the third; 2) addition of the first water to monoalkyl ammonium ions is about 2 kcal/mole more favorable for the case of monomethyl than for monobutyl and that there is relatively little change by further increases of alkyl chain length; 3) addition of the first water to monomethyl ammonium ions is about 3 kcal/mole more favorable for the case of trimethyl. Furthermore, addition of the first water to propyl ammonium is about 1.5 kcal/mole more favorable than the addition of the first water to trimethyl ammonium. - Thermodynamics, Hydration Enthalpy, Hydrophobicity, Equilibrium, Mass Spectrometry